U.S. patent application number 13/497904 was filed with the patent office on 2012-10-18 for spark plug and process for producing spark plug.
Invention is credited to Toshitaka Honda, Hirokazu Kurono, Takeshi Mitsuoka, Katsuya Takaoka, Hiroki Takeuchi, Kuniharu Tanaka.
Application Number | 20120262049 13/497904 |
Document ID | / |
Family ID | 43795602 |
Filed Date | 2012-10-18 |
United States Patent
Application |
20120262049 |
Kind Code |
A1 |
Kurono; Hirokazu ; et
al. |
October 18, 2012 |
SPARK PLUG AND PROCESS FOR PRODUCING SPARK PLUG
Abstract
A spark plug that is equipped with an insulator that has a stem
portion on a front end side, a center electrode, and a main metal
member that retains the insulator at an engaging projection
portion, wherein an inner diameter D.sub.IN of the engaging
projection portion, the maximum outer diameter d.sub.OUT of the
stem portion, which faces an inner circumferential surface of the
engaging projection portion, its inner diameter d.sub.IN, and
dielectric constant .di-elect cons. of the insulator satisfy the
condition of "(D.sub.IN-d.sub.OUT)/2.ltoreq.0.40 (mm)",
"(d.sub.OUT-d.sub.IN)/2.ltoreq.1.65 (mm)", and ".di-elect
cons..gtoreq.9.4".
Inventors: |
Kurono; Hirokazu;
(Nagoya-shi, JP) ; Honda; Toshitaka; (Nagoya-shi,
JP) ; Takaoka; Katsuya; (Ichinomiya-shi, JP) ;
Takeuchi; Hiroki; (Niwa-gun, JP) ; Tanaka;
Kuniharu; (Komaki-shi, JP) ; Mitsuoka; Takeshi;
(Konan-shi, JP) |
Family ID: |
43795602 |
Appl. No.: |
13/497904 |
Filed: |
June 23, 2010 |
PCT Filed: |
June 23, 2010 |
PCT NO: |
PCT/JP2010/004161 |
371 Date: |
March 23, 2012 |
Current U.S.
Class: |
313/141 ;
445/7 |
Current CPC
Class: |
C04B 2235/3213 20130101;
C04B 2235/963 20130101; C04B 2235/3206 20130101; C04B 2235/3225
20130101; C04B 2235/5445 20130101; H01T 13/36 20130101; C04B
2235/3418 20130101; C04B 35/117 20130101; C04B 2235/3229 20130101;
C04B 2235/3224 20130101; C04B 2235/3222 20130101; C04B 2235/3208
20130101; C04B 2235/77 20130101; C04B 2235/5436 20130101; H01T
21/02 20130101; C04B 2235/604 20130101; C04B 2235/3227 20130101;
C04B 2235/3215 20130101; H01T 13/38 20130101 |
Class at
Publication: |
313/141 ;
445/7 |
International
Class: |
H01T 13/38 20060101
H01T013/38; H01T 21/02 20060101 H01T021/02 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 25, 2009 |
JP |
2009-221066 |
Claims
1. A spark plug that is equipped with (a) an insulator that is
generally cylindrically formed to have a small-diameter stem
portion on a front end side and has a through hole passing though
in an axial direction, (b) a center electrode inserted on the front
end side of the through hole, and (c) a main metal member that is
generally cylindrically formed to have an engaging projection
portion projecting inwardly in the diametral direction and retains
the insulator inserted thereinto, at the engaging projection
portion, the spark plug being characterized by that, when an inner
diameter of the engaging projection portion is represented by
D.sub.IN (mm), the maximum outer diameter of a portion, which faces
an inner circumferential surface of the engaging projection
portion, of the stem portion is represented by d.sub.OUT (mm), and
its inner diameter is represented by d.sub.IN (mm), and dielectric
constant of the insulator is represented by .di-elect cons.,
condition (1) of "(D.sub.IN-d.sub.OUT)/2.ltoreq.0.40 (mm)",
condition (2) of "(d.sub.OUT-d.sub.IN)/2.ltoreq.1.65 (mm)", and
condition (3) of ".di-elect cons..gtoreq.9.4" are satisfied.
2. The spark plug according to claim 1, which is characterized by
that the dielectric constant .di-elect cons. is 10.5 or less, and
the insulator is such that, when a region of 250 .mu.m.times.190
.mu.m on a mirror polished surface thereof has been observed with a
magnification power of 500, an areal ratio (S.sub.A/S) of a total
area S.sub.A of pores existing in the observed region to an area S
of the observed region is 2.0 to 4.0%, and an areal ratio
(S.sub.20/S) of a total area S.sub.20 of pores, which exist in the
observed region and of which diameter defined as that of a circle
corresponding thereto is 20 .mu.m or greater, to the area S of the
observed region is 0.3% or less.
3. The spark plug according to claim 1, which is characterized by
that the insulator is formed of an alumina-based sintered body
containing Al.sub.2O.sub.3 as a main component, SiO.sub.2, oxides,
of the group 2 elements in the periodic table based on IUPAC
Recommendation 1990, of at least two kinds, including MgO and BaO,
and 0.5 to 4.0 mass % of a rare-earth element oxide such that a
total of these contents becomes 100 mass %.
4. A process for producing the spark plug according to claim 1, the
spark plug production process being characterized by comprising a
step of producing the insulator by pressing and then sintering a
raw material powder containing Al compound powder as a main
component, Si compound powder, a powder, of a compound of an
element of group 2 in the periodic table based on IUPAC
Recommendation 1990, of at least two kinds containing Mg compound
powder and Ba compound powder, and 0.5-4.0 mass % of a rare-earth
compound powder, such that sum of contents of these in terms of
oxide becomes 100 mass %.
5. The spark plug production process according to claim 4, which is
characterized by that the Si compound powder and the group 2
element compound powder have an average particle size D50 of
0.5-3.0 .mu.m, and that the pressing is conducted under a pressure
of 50-70 MPa.
Description
TECHNICAL FIELD
[0001] This invention relates to a spark plug and a process for
producing the spark plug, and in detail relates to a small spark
plug having both stain resistance and withstand voltage
characteristics for a long time, even if it is small, and a process
for producing this spark plug.
BACKGROUND TECHNOLOGY
[0002] As one of spark plugs used for ignition of internal
combustion engines, such as an automobile gasoline engine, there is
a type of spark plug in which an insulator having a stem portion of
which diameter is reduced toward the front end side through a
stepped portion is mounted on a main metal member by an engagement
of the stepped portion with an engaging projection portion formed
on an inner circumferential surface of the main metal member, and
in which a clearance is formed between the inner circumferential
surface of the engaging projection portion and an outer
circumferential surface of the stem portion, which is opposed
thereto. In such a spark plug, once the clearance is invaded by an
unburned gas generated, for example, under low-temperature
environment, etc., the outer circumferential surface of the stem
portion in the clearance is stained. As a result, stain resistance
of the spark plug is spoiled.
[0003] Thus, there is proposed a spark plug that can be designed to
have a small size without spoiling stain resistance of the spark
plug, by narrowing the clearance to have a predetermined distance
or less, as opposed to a prior art common knowledge to make the
clearance wider in view of stain resistance.
[0004] For example, in Patent Publication 1, there is described a
spark plug characterized by that it is equipped with a cylindrical
main metal member (1), an insulator (2) that is locked on an inner
circumferential side of the main metal member (1) and has an axial
hole, a center electrode (3) retained in the axial hole of the
insulator (2), and an earth electrode that forms a spark discharge
gap (g) by facing to an end of the center electrode (3), that, in
an axial direction (O) of the insulator (2), a side on which the
spark discharge gap (g) is positioned is defined as a front side,
and a side opposite to this is defined as a rear side, that the
insulator (2) has a front end portion (2i) of which diameter is
reduced by a circumferential stepped portion that is defined as an
insulator side engaging portion (2h), that the insulator is
inserted into the main metal member (1) from a rear side opening
portion, that the insulator side engaging portion (2h) is engaged
with a metal member side engaging portion (1c) that projects from
an inner circumferential surface of the main metal member (1), that
an outer circumferential surface (hereinafter referred to as a
clearance-forming outer circumferential surface) (2k) of the
portion (2i), which is positioned on the front side of the
insulator side engaging portion (2h) of the insulator (2), is
opposed to an inner circumferential surface (hereinafter referred
to as a clearance-forming inner circumferential surface) (52) of
the metal member side engaging portion (1c) in a manner to form an
engaging position clearance (Q) of a predetermined amount, and
that, when an outer diameter of the clearance-forming outer
circumferential surface (2k) is defined as d1 and when an inner
diameter of the clearance-forming inner circumferential surface
(52) is defined as D1, an amount .beta. of the engaging position
clearance, which is expressed by .beta.=(D1-d1)/2, is adjusted to
0.4 mm or less.
[0005] Furthermore, in Patent Publication 2, there is described a
spark plug characterized by that it is equipped with an insulator
that is generally cylindrically formed and has a through hole in an
axial direction, a rod-shaped center electrode that is inserted
into a front end side of the through hole of the insulator, a
generally cylindrical main metal member that retains it by
inserting a front end side in an axial direction of the insulator,
and an earth electrode that is joined at one end portion to a front
end of the main metal member, faces to the center electrode at the
other end portion opposite to the one end portion, and forms a
spark discharge gap between the other end portion and the center
electrode, that the insulator is made up of an insulator rear end
portion formed on a rear end side of the insulator, an insulator
front end portion that is formed on a front end side of the
insulator and has a diameter reduced from an outer diameter of the
insulator rear end portion, and a first insulator stepped portion
for connecting together the insular rear end portion and the
insulator front end portion, that the main metal member is made up
of a main metal member rear end portion formed on a rear end side
of the main metal member, a main metal member front end portion
that is formed on a front end side of the main metal member and
has, on at least a rear end side, a portion having an inner
diameter reduced from an inner diameter of the main metal member
rear end portion, and a first main metal member stepped portion for
connecting together the main metal member rear end portion and the
main metal member front end portion, that the first insulator
stepped portion is engaged with the first main metal member stepped
portion through a packing, that, when a section containing an axis
is viewed, an outer diameter of the insulator front end portion is
represented by d1 and an inner diameter of the main metal member
front end portion is represented by D1, and that a length, which is
parallel to the axial direction of the insulator, of a clearance
satisfying "(D1-d1)/2<0.45 mm" is 1.2 mm or more and 5 mm or
less, when the front end side of the insulator is defined as plus,
as a position on the front end side in the axial direction of
engagement positions between the packing and the main metal member
stepped portion is defined as a starting point.
[0006] By the way, in order to make a small-size spark plug, it is
effective to make the insulator thin in thickness, besides making
the clearance narrow. On the other hand, when the clearance is made
narrow, there is a tendency in general that the voltage applied to
the insulator, which forms the clearance, becomes high. Therefore,
it becomes possible to have a small-size spark plug, while
maintaining its stain resistance, by using an insulator thinned in
thickness and by making the clearance narrow. Due to a high voltage
applied to this insulator, however, insulation breakdown may occur.
As a result, withstand voltage characteristics may lower. In
particular, when a condition with a high voltage applied to the
insulator continues for a long term, withstand voltage
characteristics may lower significantly due to the occurrence of
many insulation breakdowns on the insulator. For example, when a
spark plug is made small such that nominal diameter of the mounting
thread portion becomes M12 or less, the spark plug becomes
difficult to have both stain resistance and withstand voltage
characteristics.
PRIOR ART PUBLICATIONS
Patent Publications
[0007] Patent Publication 1: Japanese Patent Application
Publication No. 2002-260817 [0008] Patent Publication 2: Japanese
Patent Application Publication No. 2005-183177
SUMMARY OF THE INVENTION
Task to be Solved by the Invention
[0009] A task to be solved by this invention is to provide a spark
plug that has both stain resistance and withstand voltage
characteristics for a long term, even if it is small.
[0010] A task to be solved by this invention is to provide a spark
plug production process capable of producing a spark plug that has
both stain resistance and withstand voltage characteristics for a
long term, even if it is small.
Means for Solving Task
[0011] This invention as a means for solving the task is a spark
plug that is equipped with an insulator that is generally
cylindrically formed to have a small-diameter stem portion on a
front end side and has a through hole passing though in an axial
direction, a center electrode inserted on the front end side of the
through hole, and a main metal member that is generally
cylindrically formed to have an engaging projection portion
projecting inwardly in the diametral direction and retains the
insulator inserted thereinto, at the engaging projection portion,
characterized by that, when an inner diameter of the engaging
projection portion is represented by D.sub.IN (mm), the maximum
outer diameter of a portion, which faces an inner circumferential
surface of the engaging projection portion, of the stem portion is
represented by dour (mm), and its inner diameter is represented by
d.sub.IN (mm), and dielectric constant of the insulator is
represented by .di-elect cons., condition (1) of
"(D.sub.IN-d.sub.OUT)/2.ltoreq.0.40 (mm)", condition (2) of
"(d.sub.OUT-d.sub.IN)/2.ltoreq.1.65 (mm)", and condition (3) of
".di-elect cons..gtoreq.9.4" are satisfied.
[0012] This invention as another means for solving the task is
characterized by that it comprises a step for producing the
insulator by conducting pressing and then sintering on a raw
material powder containing Al compound powder as a main component,
Si compound powder, a powder, of a compound of an element of group
2 in the periodic table based on IUPAC Recommendation 1990, of at
least two kinds containing Mg compound powder and Ba compound
powder, and 0.5-4.0 mass % of a rare-earth compound powder, such
that the sum of the contents of these in terms of oxide becomes 100
mass %.
Advantageous Effect of the Invention
[0013] Since a spark plug according to this invention satisfies the
conditions (1) to (3), it shows sufficient withstand voltage
characteristics without spoiling stain resistance, even if a
condition with a high voltage applied to a thinned insulator
extends to a long term. Furthermore, a spark plug production
process according to this invention produces the insulator by
pressing the raw material powder to have predetermined dimensions
and then sintering. Therefore, this insulator satisfies the
conditions (1) to (3). Thus, according to this invention, it is
possible to provide a small-size spark plug having both stain
resistance and withstand voltage characteristics for a long term,
even if it is small-sized, and a spark plug production process
capable of producing such spark plug.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a longitudinal partially sectional view showing a
spark plug according to one embodiment of this invention.
[0015] FIG. 2 is an enlarged, partial, longitudinal sectional view
showing an enlargement of an essential portion on the front end
side of a spark plug according to one embodiment of this
invention.
[0016] FIG. 3 is an enlarged, partial, longitudinal sectional view
showing an enlargement of the vicinity of a clearance portion
formed between a main metal member and an insulator in a spark plug
according to one embodiment of this invention.
MODE FOR CARRYING OUT THE INVENTION
[0017] A spark plug according to this invention is equipped with an
insulator that is generally cylindrically formed to have a
small-diameter stem portion on a front end side and has a through
hole passing though in an axial direction, a center electrode
inserted on the front end side of the through hole, and a main
metal member that is generally cylindrically formed to have an
engaging projection portion projecting inwardly in the diametral
direction and retains the insulator inserted thereinto, at the
engaging projection portion. As long as the spark plug according to
this invention is a spark plug having such constituents, other
constituents are not particularly limited, and it may have
publicly-known various constituents. For example, a spark plug
according to this invention may be equipped with the insulator, the
center electrode, the main metal member, and an earth electrode
that has one end portion joined to the main metal member and the
other end portion opposed to the center electrode to form a spark
discharge clearance between the other end portion and the center
electrode.
[0018] A spark plug according to one embodiment of this invention
is explained with reference to FIG. 1 to FIG. 3. This spark plug
100 is used as an ignition spark plug of internal combustion
engines such as automotive gasoline engines. In the following
explanation, an axis (a dash-dotted line shown in FIG. 1 and FIG.
2) of the spark plug 100 constructed to be generally rod-shaped is
referred to as "axis O". Furthermore, in FIG. 1 to FIG. 3, a lower
side of the drawing, that is, a side for installing an earth
electrode 4 is referred to as a front end side of the spark plug
100, and an upper side of the drawing, that is, a side for forming
a corrugated portion 40 is referred to as a rear end side of the
spark plug 100.
[0019] A basic structure of this spark plug 100 is explained. As
shown in FIG. 1, this spark plug 100 is equipped with an insulator
2 that is generally cylindrically formed to have a small-diameter
stem portion 30 on the front end side and has a through hole 6
passing through in the direction of the axis O, a center electrode
3 inserted on the front end side of this through hole 6, a main
metal member 1 that is generally cylindrically formed to have on an
inner circumferential surface an engaging projection portion 56
projecting inwardly in the diametral direction and retains the
insulator 2 inserted thereinto, at the engaging projection portion
56, and an earth electrode 4 having one end portion joined to the
main metal member 1 and the other end portion opposed to the center
electrode 3 to have a spark discharge clearance g between the other
end portion and the center electrode 3.
[0020] More specifically, as shown in FIG. 1, the spark plug 100 is
equipped with a generally cylindrical main metal member 1 having on
an inner circumferential surface an engaging projection portion 56
projecting inwardly in the cliametral direction to be ringlike, a
generally cylindrical insulator 2 that is inserted into this main
metal member 1 and is retained at the engaging projection portion
56 in a manner to project from the front end portion of the main
metal member 1 in the direction of the axis O, a generally
rod-shaped center electrode 3 inserted into the through hole 6 of
the insulator 2 in a manner that an electrode front end portion 36
projects from the front end of the insulator 2, and an earth
electrode 4 disposed such that one end is welded to the front end
portion of the main metal member 1 in the direction of the axis O,
that the other end side opposite to this one end side is bent
sideward, and that its side surface is disposed to be opposed to
the electrode front end portion 36 of the center electrode 3.
[0021] In the spark plug 100, as shown in FIG. 1 and FIG. 2, the
insulator 2, specifically at around a front end portion of the
after-mentioned stem portion 30, projects from the front end
surface of the main metal member 1 toward the earth electrode 4,
and the center electrode 3 at its electrode front end portion 36
projects from the front end surface of the insulator 2 toward the
earth electrode 4. As mentioned hereinafter, between the main metal
member 1 and the stem portion 30 of the insulator 2, there is
formed a base portion clearance S interposed between an inner
circumferential surface of the main metal member 1 and an outer
circumferential surface of the stem portion 30.
[0022] As shown in FIG. 1, the main metal member 1 is generally
cylindrically formed by a metal, such as low-carbon steel, to have
the engaging projection portion 56 on the inner circumferential
surface, and is used as a housing of the spark plug 100. A mounting
thread portion 7 for mounting on an engine head not shown in the
drawings is formed on an outer circumferential surface on the front
end side in the direction of the axis O in the main metal member 1.
As one example of the standard of this mounting thread portion 7,
it is possible to mention M10, M12, M14, etc. In this invention,
nominal designation of the mounting thread portion 7 means the
value defined in ISO 2705 (M12), ISO 2704 (M10), etc. Naturally, it
allows a variation within a range of dimension tolerance defined in
the standnrds. In this invention, a small-size spark plug means a
spark plug in which nominal diameter of the mounting thread portion
7 is M12 or less. On the rear end side of the mounting thread
portion 7 in the direction of the axis O, the main metal member 1
is formed with a tool engagement portion 11 with which a tool, such
as a spanner or wrench, is engaged from the outside when mounting
the main metal member 1 on the engine head. In the spark plug 100,
the tool engagement portion 11 is hexagonal in a section
perpendicular to the direction of the axis O. Furthermore, as shown
in FIG. 1, the main metal member 1 is formed with a flange portion
61 that is formed to project outwardly in the outer diametral
direction at a generally middle portion in the direction of the
axis O, on the front end side of the tool engagement portion 11 in
the direction of the axis O. Then, a gasket 10 is fitted at around
the rear end side of the mounting thread portion 7, that is, on a
seating surface 62 of the flange portion 61.
[0023] As shown in FIG. 1 and FIG. 2, the main metal member 1 is
made up of a main metal member rear end portion 54 that is formed
on the front end side of the flange portion 61 in the direction of
the axis O to be on the side of the flange portion 61, a main metal
member front end portion 53 that is formed on the front end side of
the main metal member 1 and has, on at least a rear end side, a
portion having an inner diameter reduced from an inner diameter of
the main metal member rear end portion 54, and a first main metal
member stepped portion 55 for connecting together the main metal
member rear end portion 54 and the main metal member front end
portion 53.
[0024] More specifically, as shown in FIG. 1 and FIG. 2, the main
metal member 1 is formed to have a main metal member rear end
portion 54 that is formed on the front end side of the tool
engagement portion 11 of the main metal member 1, an engaging
projection portion (also referred to as "main metal member base
portion" in this invention) 56 that is formed on the front end side
of this main metal member rear end portion 54 in the direction of
the axis O to project inwardly in the inner diametral direction of
the main metal member 1, a first main metal member stepped portion
55 for connecting together this main metal member base portion 56
and the main metal member rear end portion 54, a main metal member
front portion 58 that is formed on the front end side of the main
metal member base portion 56 in the direction of the axis O and has
an inner diameter that is nearly the same as that of the main metal
member rear end portion 54, and a second main metal member stepped
portion 57 for connecting together this main metal member front
portion 58 and the main metal member base portion 56. Therefore,
the main metal member 1 is constructed to sequentially form the
main metal member rear end portion 54, the first main metal member
stepped portion 55, the main metal member base portion 56, the
second main metal member stepped portion 57, and the main metal
member front portion 58 in this order from the flange portion 61
toward the front end side in the direction of the axis O. In this
invention, the main metal member front end portion 53 is formed of
the main metal member front portion 58, the second main metal
member stepped portion 57, and the main metal member base portion
56. The first main metal member stepped portion 55 is a main metal
member side engaging portion for an engagement with a first
insulator stepped portion 27 of the after-mentioned insulator
2.
[0025] As shown in FIG. 1 to FIG. 3, the engaging projection
portion 56 is a ring-like projection portion having an inner
diameter, which is nearly constant in the direction of the axis O,
and making a circle in a circumferential direction of an inner hole
of the main metal member 1. This engaging projection portion 56
forms a trapezoidal section, together with the first main metal
member stepped portion 55 and the second main metal member stepped
portion 57. Therefore, an inner circumferential surface 59 of the
engaging projection portion 56 extends along the axis O.
[0026] As shown in FIG. 1, the insulator 2 is a generally
cylindrical body for retaining the center electrode 3 by an
insertion thereinto. This insulator 2 has a through hole 6 that
passes through along the direction of the axis O. A generally
rod-shaped, terminal member 13 is mounted by insertion on a rear
end portion of this through hole 6 in the direction of the axis O.
A generally rod-shaped center electrode 3 is mounted by insertion
on the other end side opposite to one end side of the through hole
6, on which terminal member 13 is mounted by insertion, that is, on
the front end side of the through hole 6. As shown in FIG. 1, a
resistor 15 is disposed between the terminal member 13 and the
center electrode 3, which are mounted by insertion on the through
hole 6. Conductive glass seal layers 16 and 17 are respectively
disposed on both end portions, that is, a front end portion and a
rear end portion, of this resistor 15 in the direction of the axis
O. Then, the center electrode 3 and the terminal member 13 are
electrically connected with each other through the conductive glass
seal layers 16 and 17. In this manner, the resistor 15 and the
conductive glass seal layers 16 and 17 constitute a sintered
conductive material portion. The resistor 15 is made of a resistor
composition prepared by using, as a raw material, a mixed powder of
a glass powder, a conductive material powder and according to need
a ceramic powder other than glass. Furthermore, a high voltage
cable (not shown in FIG. 1) is connected to a rear end portion of
the terminal member 13 in the direction of the axis O through a
plug cap (not shown in FIG. 1) to apply a high voltage thereto.
[0027] In the insulator 2, as shown in FIG. 1, there is formed a
flange-shaped, projection portion 23 projecting outwardly in the
outer diametral direction from an outer circumferential surface of
the insulator 2 at a generally middle portion of the insulator 2 in
the direction of the axis O. In the insulator 2, as shown in FIG.
1, there is formed a corrugated portion 40, in which a stepped
surface containing the axis of the insulator 2 has a wavy shape, on
an outer circumferential surface on the rear end side of the
projection portion 23 in the direction of the axis O. This
corrugated portion 40 forms a wavy shape on an outer
circumferential surface of the insulator 2 to enlarge the surface
area of the outer circumferential surface of the insulator 2.
Therefore, for example, even if leaked electricity flows along the
outer circumferential surface of the insulator 2 to cause leakage
(leakage phenomenon), it is exhausted during the flow on the outer
circumferential surface of the insulator 2, and therefore an effect
of preventing leakage can be obtained.
[0028] The insulator 2 is formed of an insulator rear end portion
26 that is on the front end side of the projection portion 23 in
the direction of the axis O and extends from the projection portion
23 toward the front end side, a stem portion (also referred to as
"an insulator front end portion" in this invention) 30 that is
formed on the front end side of the insulator rear end portion 26
and has a diameter reduced from an outer diameter of this insulator
rear end portion 26, and a first insulator stepped portion 27 for
connecting together the insulator rear end portion 26 and the stem
portion 30.
[0029] More specifically, as shown in FIG. 1 and FIG. 2, the
insulator 2 is formed to have an insulator rear portion 24 that is
formed on the rear end side of the projection portion 23 in the
direction of the axis O of the insulator 2, an insulator rear end
portion 26 that is formed on the front side of the projection
portion 23, a stem portion 30 that is formed on the front end side
of the insulator rear end portion 26 in the direction of the axis
O, and a first insulator stepped portion 27 that connects together
this stem portion 30 and the insulator rear end portion 26 and
forms a circumferential stepped portion. The stem portion 30 has an
outer diameter that is smaller than that of the insulator rear end
portion 26 and is gradually reduced toward the front end side in
the direction of the axis O. That is, this stem portion 30 is
truncated cone in shape, as correctly shown in FIG. 1 and FIG.
2.
[0030] In the spark plug 100, the insulator 2 is inserted through
an opening portion on the rear end side of the main metal member 1
in the direction of the axis O, and, as shown in FIG. 1, the first
insulator stepped portion 27 of the insulator 2 is engaged with or
locked onto the first main metal member stepped portion 55 of the
main metal member 1. The first insulator stepped portion 27 is an
insular side engaging portion for an engagement with the first main
metal member stepped portion 55. As shown in FIG. 1 to FIG. 3, a
generally ringlike, plate packing 8 is disposed between the first
main metal member stepped portion 55 of the main metal member 1 and
the first insulator stepped portion 27. In this way, the first
insulator stepped portion 27 and the first main metal member
stepped portion 55 are engaged with each other through the plate
packing 8, thereby preventing the insulator 2 from being removed in
the direction of the axis O. The plate packing 8 is made of a
material high in thermal conductivity, such as copper. If the plate
packing 8 is high in thermal conductivity, heat removal from the
spark plug 100 becomes good, thereby improving heat resistance.
Such material is preferably a material having a thermal
conductivity of 200 W/mK or higher, such as copper or aluminum. In
particular, if nominal designation of the mounting thread portion 7
in the spark plug 100 is as small as M12 or less, a particularly
high heat resistance effect is shown.
[0031] In the spark plug 100, a generally ringlike packing 41,
which engages with a rear side periphery of the projection portion
23, is disposed between an opening portion inner surface on the
rear end side of the main metal member 1 in the direction of the
axis O and an outer circumferential surface of the insulator 2.
Furthermore, a generally ring-like packing 42 is disposed on the
rear side of the packing 41 with an interposal of a filler layer 9,
such as talc. Then, a swaged portion 12 is formed to hold the main
metal member 1 onto the insulator 2 by pressing the insulator 2
into the main metal member 1 toward the front end side in the
direction of the axis O and, under this condition, swaging an
opening peripheral portion of the main metal member 1 toward the
packing 42.
[0032] The center electrode 3 is fixed in an axial hole of the
insulator 2 under a condition that a front end portion of the
center electrode 3 projects from a front end surface of the
insulator 2, and it is held under insulation against the main metal
member 1. The center electrode 3 has at least at a surface layer
portion an electrode parent material 21 made of a Ni (nickel)
series alloy, such as Inconel (trade name) 600 or 601. In its
interior, there is embedded a core member 33 containing a main
component, such as Cu (copper) or Cu alloy, for accelerating heat
radiation. That is, this center electrode 3 is formed of an outer
member as a main body, and a core member 33 formed to be
concentrically embedded in an axial portion in an interior of this
outer member. The spark plug 100 equipped with the center electrode
3, in which the core member 33 is deeply embedded in the interior
as just described, is strongly against "heating" too and is
preferably used as a wide range type plug having a wide range of
use temperature.
[0033] As shown in FIG. 1, the earth electrode 4 is made of a metal
high in corrosion resistance. As one example, Ni alloy, such as
Inconel (trademark) 600 or 601, is used. This earth electrode 4 has
a generally rectangular, transverse section that is orthogonal to a
longitudinal direction of the earth electrode itself and has a
bent, square-bar-shaped external appearance. As shown in FIG. 1, a
square-rod-shaped one end portion is joined by welding or the like
to a joined portion 60 of one end portion on the front end side of
the main metal member 1 in the direction of the axis O. On the
other hand, the other end portion (also referred to as a front end
portion), which is opposite to the one end portion of this earth
electrode 4, is bent sideward to be opposed to the electrode front
end portion 36 of the center electrode 3 in the direction of the
axis O of the center electrode 3. As shown in FIG. 1 and FIG. 2,
there is formed a spark discharge gap g between opposing surfaces
of the electrode front end portion 36 of the center electrode 3 and
the earth electrode 4. In general, this spark discharge gap g is
set at 0.3 to 1.5 mm.
[0034] The thus structured spark plug 100 is characterized by, of
the stem portion 30 in the insulator 2, a portion (hereinafter may
be referred to as a stem base portion) 29, which is opposed to the
inner circumferential surface 59 of the engaging projection portion
56, and a base portion clearance S between the engaging projection
portion 56 and the stem base portion 29. The base portion clearance
S is formed, in the direction of the axis O, on the front end side
of the plate packing 8, which is disposed between the first
insulator stepped portion 27 and the first main metal member
stepped portion 55. In the spark plug 100, specifically, as
sufficiently shown in FIG. 2 and FIG. 3, when an inner diameter of
the engaging projection portion 56 is represented by D.sub.IN (mm),
the maximum outer diameter of the stem base portion 29 is
represented by d.sub.OUT (mm), and its inner diameter is
represented by d.sub.IN (mm), the stem base portion 29 and the base
portion clearance S satisfy condition (1) of
"(D.sub.IN-d.sub.OUT)/2.ltoreq.0.40 (mm)" and condition (2) of
"(d.sub.OUT-d.sub.IN)/2.ltoreq.1.65 (mm)".
[0035] In other words, the condition (1) means that the minimum
width of the base portion clearance S in the radial direction, that
is, the minimum distance between the inner circumferential surface
59 of the engaging projection portion 56 and the outer
circumferential surface of the stem base portion 29 is 0.4 (mm) or
less. If the condition (1) is satisfied, the spark plug 100 is
improved in stain resistance and heat resistance, and downsizing of
the spark plug becomes possible, too. Specifically, even if the
spark plug 100 is left under an environment of usage, which tends
to have stains, for example, at the moment of pre-delivery, it is
possible to effectively block intrusion of unburned gas into the
base portion clearance S. As a result, it is possible to prevent a
stain of the outer circumferential surface of the stem base portion
29, which is generated by adhesion of carbon or the like.
Furthermore, the stem base portion 29 and the engaging projection
portion 56 are adjacent to each other with the minimum distance.
With this, heat of the heated insulator 2 is easily transmitted
from the stem base portion 29 to the engaging projection portion 56
through the base portion clearance S. Therefore, heat removal from
the spark plug 100 is efficiently conducted, thereby improving heat
resistance of the spark plug 100. In addition, the base portion
clearance S is narrowed to satisfy the condition (1). With this,
downsizing of the spark plug 100 is also possible.
[0036] The above-mentioned "(D.sub.IN-d.sub.OUT)/2" is preferably
0.05 to 0.35 (mm), particularly preferably 0.20 to 0.30 (mm), in
view of that downsizing is possible without spoiling superior stain
resistance and heat resistance.
[0037] Herein, as shown in FIG. 1 to FIG. 3, since the stem portion
30 is generally truncated cone in shape, the outer diameter in the
stem base portion 29 is not constant in the direction of the axis
O. Thus, in this invention, the maximum outer diameter d.sub.OUT is
taken as the outer diameter of the stem base portion 29 in the
condition (1). In this invention, a portion of the stem portion 30
that is opposed to the engaging projection portion 56 does not
contain the first insulator stepped portion 27 having the plate
packing 8 interposed therebetween. Therefore, in the spark plug
100, as shown in FIG. 2 and FIG. 3, the maximum outer diameter dour
of the stem base portion 29 is the outer diameter of a portion that
is opposed to the rear end periphery in the inner circumferential
surface 59 of the engaging projection portion 56, which is in the
vicinity of the connecting portion between the first insulator
stepped portion 27 and the stem base portion 29. In other words,
the maximum outer diameter dour means the outer diameter of a
sectional contour line of the stem base portion 29 in a
hypothetical plane P.sub.1 (see FIG. 3) that contains a rear end
periphery in the inner circumferential surface 59 of the engaging
projection portion 56 and is perpendicular to the axis O.
[0038] The length of the stem base portion 29 in the direction of
the axis O, that is, the length of the inner circumferential
surface 59 in the engaging projection portion 56 in the direction
of the axis O is not particularly limited and is adjusted to, for
example, 1.2 to 5.0 mm, preferably 1.5 to 3.0 mm. If the length is
adjusted to be within the above range, it is possible to have both
stain resistance and heat resistance at even higher levels.
[0039] In other words, the condition (2) means that the maximum
thickness of the stem base portion 29 is 1.65 mm or less. If the
condition (2) is satisfied, the spark plug 100 can sufficiently be
downsized. It is preferable that the above-mentioned
"(d.sub.OUT-d.sub.IN)/2" is 1.0 mm or greater in order to maintain
strength of the stem base portion 29. As shown in FIG. 3, the
maximum thickness of the stem base portion 29 is the difference
between the outer diameter and the inner diameter of the sectional
contour lines of the stem base portion 29 in the hypothetical plane
P.sub.1 (see FIG. 3).
[0040] The spark plug 100 satisfies the following condition (3)
provided that dielectric constant of the insulator 2 is represented
by .di-elect cons.. In particular, in this invention, it is
preferable that, of the insulator 2, particularly the stem portion
30 and the stem base portion 29 satisfy the following condition
(3). Condition (3): .di-elect cons..gtoreq.9.4.
[0041] In this manner, when the insulator 2, the stem portion 30,
and stem base portion 29 (hereinafter these may be referred to as
the insulator 2 or the like.) have a dielectric constant c of the
above range, even if the voltage applied to the insulator 2 or the
like becomes high by narrowing the base portion clearance S as
mentioned above, and even if the insulator 2 or the like is thinned
as mentioned above, it shows sufficient withstand voltage
characteristics without having insulation breakdown of the
insulator 2 or the like. Furthermore, when the insulator 2 or the
like has a dielectric constant .di-elect cons. of the above range,
even if a condition that a high voltage is applied to the insulator
2 or the like continues for a long term, insulation breakdown of
the insulator 2 or the like does not occur easily, and the spark
plug 100 shows sufficient withstand voltage characteristics without
spoiling high stain resistance. Dielectric constant .di-elect cons.
of the insulator 2 is preferably 10.5 or less. If the dielectric
constant .di-elect cons. is 10.5 or less, electric field is hardly
concentrated on pores scattered in the interior of the insulator 2,
thereby effectively suppressing internal breakdown of the insulator
2 starting from the pores. Therefore, when dielectric constant
.di-elect cons. of the insulator 2 is within a range of 9.4 to
10.5, it is possible to even further effectively suppress the
occurrence of insulation breakdown. Dielectric constant .di-elect
cons. can be measured in accordance with JIS R1641.
[0042] In this way, the spark plug 100, which satisfies any of the
conditions (1) to (3), can be downsized and shows sufficient
withstand voltage characteristics without spoiling high stain
resistance, even if a condition that a high voltage is applied to
the insulator 2 or the like thinned by the downsizing continues for
a long term.
[0043] In this invention, it is preferable in the insulator 2 of
the spark plug 100 that, when a plurality of spots, for example,
nine spots, each having a region of 250 .mu.m.times.190 .mu.m on
the mirror polished surface have been observed with a magnification
power of 500, the areal ratio (S.sub.A/S) of the total area S.sub.A
of pores existing in the observed region to the area S of the
observed region is 2.0 to 4.0%. In particular, it is preferable
that the stem portion 30 or the stem base portion 29 has the areal
ratio (S.sub.A/S) of 2.0 to 4.0%. If the areal ratio (S.sub.A/S)
becomes small, electric field does not easily concentrate on pores
scattered in the interior of the insulator 2, thereby suppressing
internal breakdown of the insulator 2 starting from the pores. If
it is too small, electric field tends to rather concentrate on the
pores. This tends to generate internal breakdown of the insulator
2. Thus, in this invention, the areal ratio (S.sub.A/S) is set at
2.0% or greater to avoid the concentration of electric field on the
scattered pores. Therefore, in this invention, if the insulator 2
has the areal ratio (S.sub.A/S) of 2.0 to 4.0%, electric field is
hardly concentrated on pores, which may become starting points of
insulation breakdown, and insulation breakdown hardly occurs on the
insulator 2 or the like even if dielectric constant c of the
insulator 2 or the like is increased for the purpose of improving
withstand voltage characteristics.
[0044] The areal ratio (S.sub.A/S) is calculated as follows.
Firstly, a surface, that is, a mirror polished surface is prepared
by polishing the insulator 2 or the like to have a mirror
condition. The mirror polished surface is prepared by processing an
arbitrary surface or arbitrary section of the insulator 2 or the
like into a flat surface by using a 45 .mu.m diamond grinder and by
conducting a mirror polishing process until the surface roughness
Ra becomes around 0.01 .mu.m by sequentially using diamond pastes
of 9 .mu.m, 3 .mu.m and 0.25 .mu.m. Then, a carbon deposition is
conducted on the thus prepared, mirror polished surface for
providing conductivity. Using an electron microscope, a plurality
of spots, for example, nine spots, each having a region of 250
.mu.m.times.190 .mu.m on the mirror polished surface are observed
with a magnification power of 500, and each observed region is
photographed. Each SEM reflection electron image photograph taken
is subjected to a binarization with an image analyzing software
(Soft Imaging System "Five" made by Olympus Corporation), thereby
recognizing void portions corresponding to the pores. In each SEM
reflection electron image photograph, there are determined the
total area S.sub.A of the void portions and the areal ratio by
dividing this total area S.sub.A by the area S of the observed
region. The thus determined areal ratios are arithmetically
averaged to calculate the areal ratio (S.sub.A/S) of the insulator
2 or the like.
[0045] In this invention, when a plurality of spots, for example,
nine spots each having a region of 250 .mu.m.times.190 .mu.m on the
mirror polished surface are observed with a magnification power of
500, it is preferable in the insulator 2 of the spark plug 100 that
the areal ratio (S.sub.20/S) of the total area S.sub.20 of pores,
which exist in the observed region and of which diameter defined as
that of a circle corresponding thereto is 20 .mu.m or greater, to
the area S of the observed region is 0.3% or less. In particular,
it is preferable that the stem portion 30 or the stem base portion
29 has the areal ratio (S.sub.20/S) of 0.3% or less. In case that
the insulator 2 or the like has the areal ratio (S.sub.20/S) of
0.3% or less, relatively large pores, of which diameters defined as
those of circles corresponding thereto are 20 .mu.m or greater,
almost do not exist. Therefore, the insulator 2 or the like thinned
by downsizing of the spark plug 100 is compact, and it is possible
to maintain the original withstand voltage characteristics. In each
SEM reflection electron image photograph taken similar to that for
the areal ratio (S.sub.A/S), there are determined the total area
S.sub.20 of void portions, of which diameter converted to that of a
circle corresponding thereto exceeds 20 .mu.m, and the areal ratio
(S.sub.20/S) by dividing this total area S.sub.20 by the area S of
the observed region. The thus determined areal ratios are
arithmetically averaged to calculate the areal ratio (S.sub.20/S)
of the insulator 2 or the like.
[0046] In this invention, when the insulator 2 or the like of the
spark plug 100 has a dielectric constants of 9.4 to 10.5, and when
the areal ratio (S.sub.A/S) is 2.0 to 4.0% and when the areal ratio
(S.sub.20/S) is 0.3% or less, a synergistic effect is obtained by
the combination of the dielectric constant .di-elect cons., the
areal ratio (S.sub.A/S) and the areal ratio (S.sub.20/S). That is,
as mentioned above, electric field is not excessively concentrated
on the pores. Furthermore, in the first place, there are few pores
that become a starting point of insulation breakdown, and thereby
the spark plug 100 is capable of showing superior withstand voltage
characteristics.
[0047] To adjust the dielectric constant .di-elect cons., the areal
ratio (S.sub.A/S) and the areal ratio (S.sub.20/S) within the above
ranges in the insulator 2 of the spark plug 100, it suffices to
form the insulator 2 by an alumina-based sintered body having the
dielectric constant .di-elect cons., the areal ratio (S.sub.A/S)
and the areal ratio (S.sub.20/S) of the above ranges.
[0048] In the spark plug 100, the insulator 2 containing the stem
portion 30 is formed of an alumina-based sintered body. As its
preferable example is mentioned, this alumina-based sintered body
contains Al component as a main component, Si component, a
component, of an element of group 2 in the periodic table based on
IUPAC Recommendation 1990, of at least two kinds containing Mg
component and Sa component, and 0.5-4.0 mass % of a rare-earth
element component, such that the sum of the contents (hereinafter
may be referred to as the total content) of these becomes 100 mass
%. If the insulator 2 or the like is formed of such alumina-based
sintered body, it is capable of showing withstand voltage
characteristics without spoiling stain resistance, even if it is
thinned in thickness in order to seek downsizing of the spark plug
100.
[0049] In particular, if the alumina-based sintered body has a
dielectric constant .di-elect cons. of 9.4 to 10.5, if upon
observing the mirror polished surface, the areal ratio (S.sub.A/S)
of the total area S.sub.A of the pores existing in the observed
region to the area S of the observed region is 2.0 to 4.0%, and if
the areal ratio (S.sub.20/S) of the total area S.sub.20 of pores,
which exist in the observed region and of which diameter defined as
that of a circle corresponding thereto is 20 .mu.m or greater, is
0.3% or less, withstand voltage characteristics of the spark plug
100 equipped with the insulator 2 or the like formed of this
alumina-based sintered body are even further improved.
[0050] The above-mentioned Al component is generally alumina
(Al.sub.2O.sub.3) and exists as a main component in the
alumina-based sintered body. In this invention, "a main component"
refers to a component of which content is the highest. If Al
component is contained as a main component, the sintered body
becomes superior in withstand voltage characteristics, heat
resistance, mechanical characteristics, etc. The content of Al
component in the alumina-based sintered body is preferably 92.0
mass % to 97.0 mass %, particularly preferably 92.5 mass % to 96.5
mass %, provided that the total content is 100 mass %. If the
content of Al component is within the above range, it becomes a
compact, alumina-based sintered body. As a result, at the grain
boundary of the alumina-based sintered body, the formation of a
low-melting-point glass phase and persistence of pores are limited.
Therefore, an insulator formed of this alumina-based sintered body
shows high withstand voltage characteristics. In this invention,
the content of Al component is expressed in mass % in terms of
alumina (Al.sub.2O.sub.3), which is an oxide of Al component.
[0051] The above-mentioned Si component is a component derived from
a sintering aid and exists in the alumina-based sintered body in
the form of oxide, ion, etc. The Si component is melted upon
sintering and normally generates a liquid phase. Therefore, it
works as a sintering aid for accelerating densification of the
sintered body. Furthermore, after sintering, Si component often
forms a low-melting-point glass, etc. at the grain boundary phase
of alumina crystal grains. However, if the alumina-based sintered
body contains the after-mentioned other specific components in
addition to Si component, it tends to preferentially form a
high-melting-point glass phase, etc. other than the
low-melting-point glass, in collaboration with the other
components. Thus, the alumina-based sintered body hardly melts at
low temperatures, thereby hardly generating migration, etc. which
can be a cause of insulation breakdown. It is preferable that the
content of Si component is 1.0 to 4.0 mass %, provided that the
total content is 100 mass %. In this invention, the content of Si
component is expressed in mass % in terms of SiO.sub.2, which is an
oxide of Si component.
[0052] The above-mentioned group 2 element component is a component
derived from the sintering aid. In this invention, Mg component and
Ba component are essential components. This group 2 element
component will do by containing Mg component and Ba component. In
addition to Mg component and Ba component, it is preferable to
contain at least one, group 2 element component other than Mg
component and Ba component. Herein, as the group 2 element
component other than Mg component and Ba component, it is possible
mention Ca component and Sr component from the viewpoint of low
toxicity, etc. Specifically, as preferable group 2 element
components, it is possible to mention three kinds of Mg component,
Ba component and Ca component, three kinds of Mg component, Ba
component and Sr component, and four kinds of Mg component, Ba
component, Ca component and Sr component. In this invention,
particularly preferable group 2 element components are the
above-mentioned three kinds.
[0053] The above-mentioned Mg component is a component derived from
the sintering aid, exists in the alumina-based sintered body in the
form of oxide, ion, etc., and works as a sintering aid similar to
Si component prior to sintering. The above-mentioned Ba component,
the above-mentioned Ca component and the above-mentioned Sr
component are components derived from sintering aid, exist in the
alumina-based sintered body in the form of oxide, ion, etc., work
as sintering aids similar to Mg component prior to sintering, and
have a function of improving high-temperature strength of the
alumina-based sintered body to be obtained. Therefore, if the group
2 element component is contained in the alumina-based sintered
body, its withstand voltage characteristics and high-temperature
strength are improved, and the sintering temperature upon sintering
lowers.
[0054] The content of the group 2 element component is preferably
0.1 to 2.5 mass %, particularly preferably 0.5 to 2.9 mass %,
provided that the total content is 100 mass %, in view of that it
becomes compact to result in an alumina-based sintered body
superior in withstand voltage characteristics and high-temperature
strength when formed into an insulator, even if a raw material
powder having a relatively large particle size has been used.
[0055] Regarding each content of Mg component, Ba component, Ca
component and Sr component, it suffices that the total of these is
within the above range. In case that these components are contained
in the alumina-based sintered body, provided that, for example, the
total content is 100 mass %, the content M of Mg component is
preferably 0.01 to 0.40 mass %, the content B of Ba component is
preferably 0.1 to 1.6 mass %, particularly preferably 0.18 to 1.6
mass %, the content C of Ca component is preferably 0.2 to 0.9 mass
%, and the content S of Sr component is preferably 0.2 to 0.9 mass
%. In this invention, in case that the alumina-based sintered body
does not contain one of Ca component and Sr component, naturally
the content C or content S becomes 0 mass %. In this invention,
each content of the group 2 element components is expressed in mass
in terms of its oxide, for example, MgO, BaO, CaO or SrO. The
content of the group 2 element components refers to the total
content of each content of the group 2 element component.
[0056] The above-mentioned rare-earth element component (referred
to as RE component, too) is a component containing Sc, Y and a
lanthanoid element. Specifically, they are Sc component, Y
component, La component, Ce component, Pr component, Nd component,
Pm component, Sm component, Eu component, Gd component, Tb
component, Dy component, Ho component, Er component, Tm component,
Yb component, and Lu component. RE component exists in the
alumina-based sintered body in the form of oxide, ion, etc. Due to
containment upon sintering, this RE component is capable of
suppressing an excessive grain growth of alumina upon sintering and
increasing melting point of the grain boundary glass phase by
forming RE-Si series glass (rare-earth glass) at the grain boundary
in collaboration with Si component. With this, it is capable of
improving withstand voltage characteristics and high-temperature
strength of the alumina-based sintered body.
[0057] As RE component, each component of the above will do. La
component, Nd component, Pr component, Y component, etc. are
preferable. La component, Nd component, Pr component and Y
component, in which each element of La, Nd, Pr and Y contained in
those has a large radius, are considered to form a
high-melting-point crystalline phase in cooperation with Si
component and to easily form hexaaluminate crystals having a high
melting point of about 2000.degree. C. in collaboration with Al
component. These hexaaluminate crystals have .beta.-alumina
structure and have as a compositional formula a composition
represented by RE(2A)x(Al)yOz (the above-mentioned x, y and z are
respectively defined as x=0-2.5, y=11-16, and z=18-28. "2A"
represents a group 2 element in the periodic table based on IUPAC
Recommendation 1990.). For example, it is a crystalline phase
represented by a chemical formula of REAl.sub.11O.sub.18, etc.
Therefore, if the alumina-based sintered body contains as RE
component at least one component selected from the group consisting
of La component, Nd component, Pr component and Y component, a
high-melting-point hexaaluminate crystals are formed. With this,
the insulator 2 formed of the alumina-based sintered body shows
withstand voltage characteristics and high-temperature strength. It
suffices that the hexaaluminate crystals exist in the alumina-based
sintered body. Although their existence spots are not particularly
limited, it is preferable that they exist into the interior of the
alumina-based sintered body. It is particularly preferable that
they exist at the grain boundary between two grains and/or triple
point of alumina crystalline grains.
[0058] The existence of the hexaaluminate crystals can be
identified by X-ray diffraction, for example, by using JCPDS cards.
Since no JCPDS cards exist with respect to Pr component and Nd
component, identification by X-ray diffraction is impossible
directly. Ionic radii of Pr.sup.3+ and Nd.sup.3+ are, however,
almost equivalent to ionic radius of La.sup.3+. Therefore,
hexaaluminate crystals containing Pr component and Nd component
show an X-ray diffraction spectrum analogous to JCPDS card (No.
33-699) of hexaaluminate crystals containing La component.
Therefore, it is possible to verify the existence of hexaaluminate
crystals containing Pr component and Nd component by a comparison
with JCPDS card of hexaaluminate crystals containing La component.
If the hexaaluminate crystals are formed by deposition in the
burning process, anisotropic growth of the grains upon burning
hardly occurs. Therefore, it is preferable.
[0059] As the content of RE component becomes high, there is a
tendency that dielectric constant .di-elect cons. of the
alumina-based sintered body becomes high. On the other hand, it may
not become a compact alumina-based sintered body without making the
sintering temperature high. Therefore, the content of RE component
of the alumina-based sintered body is suitably adjusted in view of
the formation of hexaaluminate crystals, dielectric constant
.di-elect cons. of the alumina-based sintered body, and its
sinterability. For example, it is preferable that the content of RE
component is 0.5 to 4.0 mass %, provided that the above-mentioned
total content is 100 mass %. In this invention, the content of RE
component in the alumina-based sintered body is expressed in mass %
in terms of oxide of each component. Specifically, in case that RE
components are La component, Nd component and Y component, they are
expressed in mass % in terms of oxide, "RE.sub.2O.sub.3". In the
case of being Pr component, it is expressed in mass % in terms of
oxide, "Pr.sub.6O.sub.11". When the alumina-based sintered body
contains a plurality of RE components, the content of RE components
is the total of the content of every RE component.
[0060] In this invention, it is possible to measure each content of
Al component, Si component, the group 2 element component, and RE
component in mass % in terms of oxide by quantitative analysis,
fluorescence X-ray analysis or chemical analysis using, for
example, electron probe micro analyzer (EPMA) or energy dispersive
micro analyzer (EPMA/EDS). In this invention, the results
calculated by conducting the above-mentioned quantitative analysis,
fluorescence X-ray analysis, or chemical analysis on the
alumina-based sintered body almost agree with the mixing ratio of
the raw material powder used for producing the alumina-based
sintered body.
[0061] Therefore, as a quantitative analysis is conducted using,
for example, an electron probe micro analyzer, the alumina-based
sintered body contains Al.sub.2O.sub.3 as a main component,
SiO.sub.2, oxides, of the group 2 elements in the periodic table
based on IUPAC Recommendation 1990, of at least two kinds,
including MgO and BaO, and 0.5 to 4.0 mass % of a rare-earth
element oxide such that the total of these contents becomes 100
mass %.
[0062] The alumina-based sintered body consists essentially of Al
component, Si component, the group 2 element component, and RE
component. Herein, "essentially" means not allowing a component
other than the above-mentioned components to be contained
positively by addition, etc. Therefore, the alumina-based sintered
body may contain inevitable impurities to the extent of not
spoiling an object of this invention. As such inevitable
impurities, it is possible to mention, for example, Na, S, N, etc.
It is preferable that the content of these inevitable impurities is
smaller. For example, it is preferably 0.1 parts by mass or less,
provided that the total mass of Al component, Si component, the
group 2 element component, and RE component. Furthermore, besides
the inevitable impurities, the alumina-based sintered body may
contain small amounts of other components, for example, B
component, Ti component, Mn component, Ni component, etc., in
addition to the above-mentioned Al component, Si component, the
group 2 element component, and RE component.
[0063] In the spark plug 100, the insulator 2 is formed of the
alumina-based sintered body. The insulator 2 and the alumina-based
sintered body have the above-mentioned composition and the
above-mentioned characteristics of the same. Therefore, according
to this invention, it is possible to provide a spark plug that is
equipped with both stain resistance and withstand voltage
characteristics for a long term, even if it is small-sized.
Furthermore, according to this invention, it is possible to provide
a small-sized spark plug that is equipped with both stain
resistance and withstand voltage characteristics for a long term,
even if mounted on a high-power internal combustion engine,
etc.
[0064] A spark plug production process according to this invention
comprises a step of producing an insulator by pressing and then
sintering a raw material powder containing Al compound powder as a
main component, Si compound powder, a powder, of a compound of an
element of group 2 (hereinafter may be referred to as group 2
element compound powder) in the periodic table based on IUPAC
Recommendation 1990, of at least two kinds, containing Mg compound
powder and Ba compound powder, and 0.5-4.0 mass % of a rare-earth
compound powder, such that the sum of the contents of these in
terms of oxide becomes 100 mass %. In the following, a spark plug
production process according to this invention is specifically
explained.
[0065] In a spark plug production process according to this
invention, firstly, a raw material powder, that is, Al compound
powder, Si compound powder, at least two kinds of group 2 element
compound powders containing Mg compound powder and Ba compound
powder, and a rare-earth compound powder are mixed together in a
slurry. In some cases, every powder of the same material as the Al
component, the same material as the Si component, the same material
as the group 2 element component, and the same material as the RE
component (these powers can also be referred to as the raw material
powders.) is mixed in a slurry. Herein, it is possible to set the
mixing percentage of each powder, for example, to be the same as
the content of each component. It is preferable to conduct this
mixing for eight hours or longer to make the mixing condition of
the raw material powder homogeneous and to make the sintered body
obtained highly compact.
[0066] Al compound power is not particularly limited so long as it
is a compound that is converted to Al component by sintering.
Generally, alumina (Al.sub.2O.sub.3) powder is used. Al compound
powder may contain inevitable impurities in reality, such as Na,
etc. Therefore, it is preferable to use one having a high purity.
For example, it is preferable that purity of Al compound powder is
99.5% or higher. As Al compound powder, it is preferable in general
to use a powder having an average particle size of 0.1-5.0 .mu.m to
obtain a compact alumina-based sintered body. Herein, the average
particle size refers to the value measured by laser diffractometry
by Microtrac particle size distribution measurement device
(MT-3000) made by NIKKISO CO., LTD.
[0067] Si compound powder is not particularly limited so long as it
is a compound that is converted to Si component by sintering. For
example, it is possible to mention various inorganic powders such
as oxides (including complex oxides), hydroxides, carbonates,
chlorides, sulfates, nitrates, phosphates, etc. of Si.
Specifically, it is possible to mention SiO.sub.2 powder, etc. In
the case of using a powder other than oxides as Si compound powder,
its usage is grasped in mass % in terms of oxide. Purity of Si
compound powder is basically the same as that of Al compound
powder. It is preferable that Si compound powder has an average
particle size D50 of 0.5-3.0 .mu.m. If Si compound powder has an
average particle size D50 in the above range, it is possible to
make the grinding time of Si compound powder relatively short. This
is superior in productivity. In particular, it is possible to
suppress the generation of pores, of which diameter defined as that
of a circle corresponding thereto is 20 .mu.m or greater.
Specifically, as the average particle size D50 is made large, pores
of which diameter defined as that of a circle corresponding thereto
is 20 .mu.m or greater tend to be generated. With this, the
above-mentioned areal ratio (S.sub.20/S) tends to become large. The
above-mentioned average particle size D50 is defined as a particle
size of an integrated value of 50% in the particle size
distribution and refers to the value measured by laser
diffractometry by Microtrac particle size distribution measurement
device (MT-3000) made by NIKKISO CO., LTD.
[0068] The above-mentioned group 2 element compound powder is not
particularly limited so long as it is a compound that is converted
to at least two, group 2 element components containing Mg component
and Ba component. For example, it is possible to mention various
inorganic powders such as oxides (including complex oxides),
hydroxides, carbonates, chlorides, sulfates, nitrates, phosphates,
etc. of alkali-earth elements. The group 2 element compound powder
is preferably made up of Mg compound powder, Ba compound powder,
and Ca compound powder and/or Sr compound powder. Specifically, it
is possible to mention MgO powder and MgCO.sub.3 powder as Mg
compound powder, BaO powder and BaCO.sub.3 powder as Ba compound
powder, CaO powder and CaCO.sub.3 powder as Ca compound powder, and
SrO powder and SrCO.sub.3 powder as Sr compound powder, and so on.
In the case of using a powder other than oxides as the group 2
element compound powder, its usage is grasped in mass % in terms of
oxide. Purity of the group 2 element compound powder is basically
the same as Al compound powder. By the same reason as that for Si
compound powder, it is preferable that the average particle size
D50 of the group 2 element compound powder is 0.5-3.0 .mu.m.
[0069] The rare-earth element compound powder is not particularly
limited so long as it is a compound that is converted to RE
component by sintering. For example, it is possible to mention
powders of rare-earth element oxides and its complex oxides, etc.
By adjusting the mixing proportion of the rare-earth element
compound power in the raw material powder, it is possible to adjust
dielectric constant E of the aluminum-based sintered body to be
obtained. Specifically, there is a tendency that the dielectric
constant .di-elect cons. increases by increasing the mixing
proportion of the rare-earth element compound powder. In the case
of using a powder other than oxides as the rare-earth element
compound powder, its usage is grasped in mass % in terms of oxide.
Purity and the average particle size of the rare-earth element
compound powder are basically the same as those of Al compound
powder.
[0070] In the raw material powder, each powder is mixed such that
the content of the rare-earth compound powder in terms of oxide in
the raw material powder becomes 0.5-4.0 mass %. In a spark plug
production process according to this invention, the content of each
of Al compound powder, Si compound powder, the group 2 element
compound powder, the rare-earth compound powder, etc. in terms of
oxide is basically the same as the content of each in the
alumina-based sintered body.
[0071] It is also possible to add, for example, a hydrophilic
binder as a binder to the raw material powder. As this hydrophilic
binder, it is possible to mention, for example, polyvinyl alcohol,
water-soluble acrylic resins, gum arabic, dextrin, etc.
Furthermore, as a solvent for dispersing the raw material powder,
it is possible to use, for example, water, alcohol, etc. These
hydrophilic binder and solvent can be used singly or in a
combination of at least two kinds. Regarding the usage of the
hydrophilic binder and the solvent, when the raw material powder is
100 parts by mass, the hydrophilic binder is 0.1-5.0 parts by mass,
preferably 0.5-3.0 parts by mass, and water used as the solvent is
40-120 parts by mass, preferably 50-100 parts by mass.
[0072] The thus obtained slurry can be adjusted, for example, to
have an average particle size of 1.4-5 .mu.m. Then, the thus
obtained slurry is granulated to have an average particle size of
50-200 .mu.m, preferably 70-150 .mu.m by spray drying by a spray
dry method or the like. Each average particle size refers to the
value measured by laser diffractometry (Microtrac particle size
distribution measurement device (MT-3000) made by NIKKISO CO.,
LTD.).
[0073] This granulated substance is pressed to obtain an unbaked
compact preferably having the shape and the size of the insulator
2. This pressing is conducted under a pressure of 50-70 MPa. If the
pressure is in the range, it is possible to adjust the areal ratio
(S.sub.A/S) in the alumina-based sintered body obtained to
2.0-4.0%. Specifically, the areal ratio (S.sub.A/S) increases by
lowering the pressure, and in contrast the areal ratio (S.sub.A/S)
decreases by heightening the pressure. The obtained unbaked compact
is ground to trim its own shape. This unbaked compact is formed of
a granulated substance having a relatively large average particle
size. Therefore, it is superior in machinability and can easily be
shaped into a desired shape with a high productivity by the above
method with an industrially low price.
[0074] The unbaked compact thus ground and shaped into a desired
shape is baked in an atmosphere of the air at 1500-1700.degree. C.,
preferably 1550-1650.degree. C., for 1-8 hours, preferably 3-7
hours, to obtain an alumina-based sintered body. If the baking
temperature is 1500-1700.degree. C., the sintered body is easily
sufficiently densified. This makes it difficult to generate an
abnormal grain growth of alumina component. Therefore, it is
possible to secure withstand voltage characteristics and mechanical
strength of the alumina-based sintered body to be obtained.
Furthermore, if the baking time is 1-8 hours, the sintered body is
easily sufficiently densified. This makes it difficult to generate
an abnormal grain growth of alumina component. Therefore, it is
possible to secure withstand voltage characteristics and mechanical
strength of the alumina-based sintered body to be obtained.
[0075] By thus sintering the unbaked compact having the above
composition, there is obtained an alumina-based sintered body
containing Al component as a main component, Si component, the
group 2 element component of at least two kinds containing Mg
component and Ba component, and 0.5-4.0 mass % of a rare-earth
element component. This alumina-based sintered body has a
dielectric constant c within a range of 9.4-10.5. Furthermore, the
alumina-based sintered body obtained is such that, when a plurality
of spots, for example, nine spots, each having a region of 250
.mu.m.times.190 .mu.m on the mirror polished surface have been
observed with a magnification power of 500, the areal ratio
(S.sub.A/S) of the total area S.sub.A of pores existing in the
observed region to the area S of the observed region is 2.0 to
4.0%, and the areal ratio (S.sub.20/S) of the total area S.sub.20
of pores, which exist in the observed region and of which diameter
defined as that of a circle corresponding thereto is 20 .mu.m or
greater, to the area S of the observed region is 0.3% or less.
Therefore, this alumina-based sintered body shows a high stain
resistance and high withstand voltage characteristics for a long
term, when it has been formed into an insulator of a small-sized
spark plug. Consequently, this alumina-based sintered body is
preferable as a material that forms an insulator, with which a
small-sized spark plug or a spark plug for a high-power internal
combustion engine is equipped, and particularly as a material that
forms an insulator with which a small-sized spark plug having both
a high stain resistance and high withstand voltage characteristics
for a long term is equipped.
[0076] According to need, this alumina-based sintered body may be
shaped again in its shape or the like to agree with the shape and
the size of the insulator 2. In this manner, it is possible to
produce an alumina-based sintered body and the insulator 2 for the
spark plug 100, which is constituted of this alumina-based sintered
body.
[0077] Next, the center electrode 3 is installed in the obtained
insulator 2 by an insertion into its through hole 6. The insulator
2, in which the center electrode 3 has been installed, is inserted
into the main metal member 1, and the first main metal member
stepped portion 55 and the first insulator stepped portion 27 are
engaged with each other, thereby attaching the insulator 2 to the
main metal member 1. The main metal member 1 has been adjusted to
the above-mentioned shape and size. The earth electrode 4 is joined
to around an end portion of the main metal member 1 by electric
resistance welding or the like before or after attaching the
insulator 2. Thus, it is possible to produce the spark plug 100
satisfying the above conditions (1) to (3). In the spark plug
production process according to this invention, it is possible to
mention one embodiment of a spark plug according to this invention,
which is shown, for example, in FIG. 1 to FIG. 3, as a mode for
assembling the center electrode, the insulator, and the main metal
member.
[0078] A spark plug according to this invention is used as an
ignition plug of automotive internal combustion engines, such as
gasoline engines, diesel engines, etc., and is fixed at a
predetermined position by subjecting the mounting thread portion 7
to a threaded engagement with a threaded hole provided with a head
(not shown in the drawings) that delimits and forms a combustion
chamber of an internal combustion engine.
[0079] Thus, the insulator 2 is made of an alumina-based sintered
body having the above-mentioned composition and the above-mentioned
characteristics. Therefore, according to this invention, it is
possible to provide a spark plug production process that is capable
of producing a spark plug equipped with both stain resistance and
withstand voltage characteristics for a long term, even if it is
small-sized. Furthermore, according to this invention, it is
possible to provide a process for producing a small-sized spark
plug that is equipped with both stain resistance and withstand
voltage characteristics for a long term, even if it is installed in
a high power, internal combustion engine or the like.
[0080] A spark plug according to this invention is not limited to
the above-mentioned embodiment and can be variously modified to the
extent that the object of the present invention can be achieved.
For example, in the spark plug 100, the stem portion 30 is
generally truncated cone in shape. In this invention, however, the
stem portion may be equipped with a cylindrical, stem base portion
having a generally uniform outer diameter, and, from this stem base
portion through a stepped portion, a stem front end portion that
has a diameter smaller than that of the stem base portion and is
generally truncated cone in shape.
[0081] The spark plug 100 is equipped with the center electrode 3
and the earth electrode 4. In this invention, however, it may be
equipped with a noble metal chip at the front end portion of the
center electrode and/or on the surface of the earth electrode. The
noble metal chip, which is formed at the front end portion of the
center electrode and on the surface of the earth electrode,
generally has a cylindrical shape, is adjusted to have suitable
dimensions, and is welded onto the front end portion of the center
electrode or the surface of the earth electrode by a suitable
welding method, such as laser welding or electric resistance
welding. The spark discharge clearance is formed between the
surface of the noble metal chip formed at the front end portion of
the center electrode and the surface of the noble metal chip formed
on the surface of the earth electrode. As a material forming this
noble metal chip, it is possible to mention a noble metal, such as
Pt, Pt alloy, Ir, Ir alloy, etc.
EXAMPLES
Examples 1-15 and Comparative Examples 1-5
[0082] A slurry was prepared by adding a hydrophilic binder to a
raw material powder of alumina powder, Si compound powder, Mg
compound powder, Ba compound powder, Ca compound powder and/or Sr
compound powder as the group 2 element compound powder, and a
rare-earth compound powder (the type of each powder mixed is shown
in Table 1). The average particle size D50 of the Si compound
powder and the group 2 element compound powder is shown in Table 1
as "the average particle size D (.mu.m) of the sintering aid"
50.
[0083] The obtained slurry was granulated into a powder having an
average particle size of about 100 .mu.m by spray drying by a spray
dry method or the like. This powder was subjected to a rubber
pressing by a compacting pressure shown in Table 1 to obtain an
unbaked compact. This unbaked compact was baked under an atmosphere
of the air by setting the sintering time at 1 to 8 hours within a
sintering temperature range of 1500-1700.degree. C., followed by
glazing and a finish sintering, thereby obtaining each
alumina-based sintered body. The sintering conditions were all set
at the same conditions within the above-mentioned ranges.
[0084] Each composition, that is, the content of each component of
the obtained alumina-based sintered bodies was calculated as mass
percentage (%) by a quantitative analysis using an energy
dispersive microanalyzer (EPMA/EDS), provided that the total
content (in terms of oxide) of Al component, Si component, the
group 2 element component, and the rare-earth element component is
100 mass %. As the energy dispersive microanalyzer (EPMAIEDS)
analysis condition, the arithmetic mean value when measuring ten
points was determined by using a field emission electron probe
microanalyzer (JXA-8500F, made by JEOL Ltd.) and setting spot
diameter .phi. at 200 and accelerating voltage at 20 kV. The
results are shown in Table 1. In Table 1, a blank column means that
the component was not detected. The content of each component shown
in Table 1 almost agreed with the mixing percentage in the raw
material powder. Furthermore, there are shown in Table 2 the
results obtained by measuring or calculating the dielectric
constant .di-elect cons., the areal ratio (S.sub.A/S), and the
areal ratio (S.sub.20/S) of each alumina-based sintered body
obtained, by the above-mentioned methods.
TABLE-US-00001 TABLE 1 Table 1 Compacting D50 of Plug Pressure
Sintering Composition (mass % in terms of oxide) Type (MPa) Aid
(.mu.m) Al.sub.2O.sub.3 SiO.sub.2 CaO MgO BaO SrO La.sub.2O.sub.3
Nd.sub.2O.sub.3 Pr.sub.6O.sub.11 Y.sub.2O.sub.3 Example 1 M12 48
3.8 94.50 2.80 0.40 0.10 0.70 1.50 Example 2 M12 48 3.8 94.50 2.80
0.40 0.10 0.70 1.50 Example 3 M12 48 3.8 94.50 2.80 0.40 0.10 0.70
1.50 Example 4 M12 75 2.0 94.50 2.80 0.40 0.10 0.70 1.50 Example 5
M12 70 2.0 94.50 2.80 0.40 0.10 0.70 1.50 Example 6 M12 60 3.8
94.50 2.80 0.40 0.10 0.70 1.50 Example 7 M12 50 3.0 94.50 2.80 0.40
0.10 0.70 1.50 Example 8 M12 55 3.0 93.60 2.80 0.40 0.10 0.70 2.40
Example 9 M12 50 3.0 92.80 2.30 0.40 0.10 0.40 4.00 Example 10 M12
52 3.0 92.50 2.10 0.30 0.10 0.30 4.70 Example 11 M12 48 3.8 94.60
3.00 0.20 0.70 1.50 Example 12 M12 48 3.8 94.50 2.80 0.10 0.70 0.40
1.50 Example 13 M12 48 3.8 94.50 2.80 0.40 0.10 0.70 1.50 Example
14 M12 48 3.8 94.50 2.80 0.40 0.10 0.70 1.50 Example 15 M12 48 3.8
94.50 2.80 0.40 0.10 0.70 1.50 Com. Ex. 1 M14 48 3.8 94.50 2.80
1.50 0.50 0.70 Com. Ex. 2 M14 48 3.8 94.50 2.80 1.50 0.50 0.70 Com.
Ex. 3 M12 48 3.8 94.50 2.80 1.50 0.50 0.70 Com. Ex. 4 M12 48 3.8
94.50 2.80 1.50 0.50 0.70 Com. Ex. 5 M10 48 3.8 94.50 2.80 1.50
0.50 0.70
[0085] (Stain resistance) Using each alumina-based sintered body,
the spark plug 100 shown in FIG. 1 to FIG. 3 was produced. In the
spark plugs of Examples 1-15 and Comparative Examples 1-5, nominal
diameter of the mounting thread portion 7 was adjusted to the value
conforming to the plug type shown in Table 1, the spark discharge
clearance g was adjusted to 1.1 mm, and the inner diameter D.sub.IN
(mm) of the engaging projection portion 56 and the maximum outer
diameter d.sub.OUT (mm) and the inner diameter d.sub.IN (mm) of the
stem base portion were respectively adjusted to the values shown in
Table 1. Using each spark plug thus prepared, a pre-delivery
durability test was conducted. That is, each spark plug was mounted
on a test car (displacement: 1500 cc, in-line four) with a voltage
application polarity by setting the earth electrode 4 positive and
the center electrode 2 negative. A travelling pattern (test room
temperature: -10.degree. C.) exemplified in JIS:D 1606 was defined
as one cycle, and this was repeated until insulating resistance of
the spark plug lowered to 10 M.OMEGA. or less. Stain resistance of
the spark plug was evaluated by the number of the cycles. It was
judged as "O" in case that the number of the cycles was 10 cycles
or greater. It was judged as "x" in case that it was 6 cycles or
less. The results are shown in Table 2.
[0086] (Withstand voltage characteristics: applied voltage 33 kV)
Similar to stain resistance, each spark plug of Examples 1-15 and
Comparative Examples 1-5 was prepared by a plural number. A voltage
of 33 kV was continuously applied for 200 hours between the main
metal member 1 and the center electrode 3 of a spark plug prepared
under an environment having a surrounding pressure of 10 MPa. This
test was conducted on each spark plug. There was calculated the
breakdown percentage (%) of the number of spark plugs with the
occurrence of insulation breakdown to the total number of each
spark plug tested, to evaluate withstand voltage characteristics of
the spark plugs. Withstand voltage characteristics were judged as
"{circle around (o)}" in case that the breakdown percentage (%) was
0%, "O" in case that the breakdown percentage (%) was greater than
0% and less than 20%, and "x" in case that the breakdown percentage
(%) was 20% or greater. The results are shown in Table 2 as
"withstand voltage characteristics, applied voltage 33 kV". In
withstand voltage characteristics by applying a voltage of 33 kV to
the main metal member 1 and the center electrode 3, it is
practically admissible if the breakdown percentage (%) is less than
20%, and it shows sufficient withstand voltage characteristics in
practice for a long term if the breakdown percentage (%) is 0%.
[0087] (Withstand voltage characteristics: applied voltage 38 kV)
Withstand voltage characteristics upon applying a high voltage in
each spark plug were evaluated in a manner that is basically the
same as that for the above-mentioned withstand voltage
characteristics 1, except in that the voltage continuously applied
between the main metal member 1 and the center electrode 3 was set
at 38 kV. The results are shown in Table 2 as "withstand voltage
characteristics, applied voltage 38 kV" In withstand voltage
characteristics by applying a voltage of 38 kV to the main metal
member 1 and the center electrode 3, it is practically admissible
if the breakdown percentage (%) is less than 20%, and it shows
sufficient withstand voltage characteristics in practice for a long
term if the breakdown percentage (%) is 0%.
[0088] Dielectric constant 6, the areal ratio (S.sub.A/S) and the
areal ratio (S.sub.20/S) in the insulator of each spark plug were
the same as those of the alumina-based sintered body shown in Table
2.
TABLE-US-00002 TABLE 2 Table 2 Withstand Voltage Characteristics
Applied Applied D.sub.IN d.sub.OUT d.sub.IN (D.sub.IN -
d.sub.OUT)/2 (d.sub.OUT - d.sub.IN)/2 (S.sub.A/S) (S.sub.20/S)
Stain Voltage Voltage (mm) (mm) (mm) (mm) (mm) .epsilon. (%) (%)
Resistance 33 kV 38 kV Example 1 6.2 5.6 2.3 0.40 1.65 9.4 4.7 0.5
.largecircle. .circleincircle. .largecircle. Example 2 6.2 5.6 2.3
0.30 1.65 9.4 4.7 0.5 .largecircle. .circleincircle. .largecircle.
Example 3 6.2 5.6 2.3 0.40 1.55 9.4 4.7 0.5 .largecircle.
.circleincircle. .largecircle. Example 4 6.2 5.6 2.3 0.30 1.65 9.4
1.5 0.2 .largecircle. .circleincircle. .largecircle. Example 5 6.2
5.6 2.3 0.30 1.65 9.4 2.0 0.2 .largecircle. .circleincircle.
.circleincircle. Example 6 6.2 5.6 2.3 0.30 1.65 9.4 2.9 0.5
.largecircle. .circleincircle. .largecircle. Example 7 6.2 5.6 2.3
0.30 1.65 9.4 4.0 0.3 .largecircle. .circleincircle.
.circleincircle. Example 8 6.2 5.6 2.3 0.30 1.65 10.0 3.8 0.3
.largecircle. .circleincircle. .circleincircle. Example 9 6.2 5.6
2.3 0.30 1.65 10.5 4.0 0.3 .largecircle. .circleincircle.
.circleincircle. Example 10 6.2 5.6 2.3 0.30 1.65 11.0 3.9 0.3
.largecircle. .circleincircle. .largecircle. Example 11 6.2 5.6 2.3
0.30 1.65 9.4 4.7 0.5 .largecircle. .circleincircle. .largecircle.
Example 12 6.2 5.6 2.3 0.30 1.65 9.4 4.7 0.5 .largecircle.
.circleincircle. .largecircle. Example 13 6.2 5.6 2.3 0.30 1.65 9.4
4.7 0.5 .largecircle. .circleincircle. .largecircle. Example 14 6.2
5.6 2.3 0.30 1.65 9.4 4.7 0.5 .largecircle. .circleincircle.
.largecircle. Example 15 6.2 5.6 2.3 0.30 1.65 9.4 4.7 0.5
.largecircle. .circleincircle. .largecircle. Com. Ex. 1 7.9 6.9 2.7
0.50 2.10 9.0 4.8 0.5 X .circleincircle. .circleincircle. Com. Ex.
2 7.9 7.4 2.7 0.25 2.35 9.0 4.8 0.5 .largecircle. .circleincircle.
.circleincircle. Com. Ex. 3 6.2 5.2 2.3 0.50 1.45 9.0 4.8 0.5 X
.circleincircle. X Com. Ex. 4 6.2 5.6 2.3 0.40 1.65 9.0 4.8 0.5
.largecircle. .circleincircle. X Com. Ex. 5 5.2 4.8 1.7 0.20 1.55
9.0 4.8 0.5 .largecircle. .circleincircle. X
[0089] As shown in Table 2, even though Examples 1-15 satisfying
all of the above-mentioned conditions (1) to (3) were small-sized
spark plugs such that nominal diameters of the mounting thread
portion 7 were M12, insulation breakdown hardly occurred, and high
withstand voltage characteristics were shown, without spoiling
stain resistance, not only in the case of continuously applying a
voltage of 33 kV for a long term of 200 hours, but also in the case
of continuously applying a high voltage of 38 kV for a long term of
200 hours. Similarly, even a small-sized spark plug showed high
withstand voltage characteristics without spoiling stain
resistance, if the alumina-based sintered body contains
Al.sub.2O.sub.3 as a main component, SiO.sub.2, oxides, of group 2
element of the periodic table based on IUPAC Recommendation 1990,
of at least two kinds, containing MgO and BaO, 0.5-4.0 mass % of a
rare-earth element oxide, provided that the total of these contents
becomes 100 mass %.
[0090] Furthermore, superior withstand voltage characteristics were
shown for a long term by each of Examples in which dielectric
constant .di-elect cons. was 9.4-10.5, the areal ratio (S.sub.A/S)
was 2.0-4.0%, and the areal ratio (S.sub.20/S) was 0.3% or lower,
and Examples in which alumina-based sintered bodies were produced
so that the average particle size D50 of the raw material powder
was 0.5-3.0 .mu.m and the compacting pressure was 50-70 MPa. In
particular, in each of Examples 5 and 7-9 satisfying all of these,
no insulation breakdown occurred at all and extremely superior
withstand voltage characteristics were shown for a long term, even
if a voltage of 33 kV and a high voltage of 38 kV are continuously
applied for a long term of 200 hours.
[0091] On the other hand, since Comparative Examples 1 and 2 do not
satisfy the condition (2) and have very thick, stem base portions
of 2.1 mm or greater, a sufficient downsizing is not possible,
although they are superior in withstand voltage characteristics.
Comparative Examples 3-5 satisfy the condition (2), but do not
satisfy the condition (3). Therefore, insulation breakdown tended
to occur, and withstand voltage characteristics when applying a
high voltage of 38 kV did not reach a practical admissible range.
Comparative Examples 1 and 3 did not satisfy the condition (1) and
were inferior in stain resistance since the base portion clearance
S was wide.
INDUSTRIAL APPLICABILITY
[0092] A spark plug according to this invention can be used for any
internal combustion engines. It is suitable as a small-sized spark
plug that requires a thinned insulator, for example, as a spark
plug used for high-power internal combustion engines, etc. In
particular, it is suitable as a spark plug used for internal
combustion engines requiring a small-sized spark plug equipped with
both stain resistance and withstand voltage characteristics for a
long term.
EXPLANATIONS OF SIGNS
[0093] 100: spark plug, 1: main metal member, 2: insulator, 3:
center electrode, 4: earth electrode, 6: through hole, 7: mounting
thread portion, 29: stem base portion, 30: stem portion (insulator,
reduced diameter portion), 56: engaging projection portion (main
metal member base portion), g: spark discharge clearance, and S:
base portion clearance.
* * * * *